Modular buildings have become increasingly popular in high-growth areas of the country for various purposes. Rapidly expanding school districts often employ modular buildings, especially trailer-type buildings, for temporary classrooms. Prisons employ modular buildings to house inmates in an overflow situation. Expanding businesses often use modular buildings to provide temporary office space. Modular buildings are also employed as temporary housing in disaster areas.
Recently, there has been increased demand for more substantial construction in modular buildings. High density applications suggest the desirability for multi-level or storied modular buildings. Prison population overflow situations in particular require sturdier and more secure construction so as to deter vandalism and escape. In some applications, modular buildings become permanent by design due to speed of construction. In other cases, modular buildings become permanent by default, as when budgetary and other pressures on an institution (such as a school district) preclude further outlays for new facilities.
In order to be readily transportable, a prefabricated modular building construction component such as a floor or roof must be sized so that it can be transported along a highway. Furthermore, it must not be so heavy that it cannot be handled by conventional diesel-powered semi-tractors and moved as relatively conventional freight. However, the competing requirements of size and weight versus strength and durability has led to a number of undesirable compromises in the construction of modular buildings.
One example of a modular building is shown in U.S. Pat. No. 5,113,625 to Davis. A modular building constructed in accordance with this patent includes a support frame with a connected concrete pan which allows a concrete floor to be directly poured into the frame. Although the integral pan allows a concrete floor to be poured directly into the frame, the floor is relatively thin and the only longitudinal support is provided by a pair of narrow exterior framing members. Furthermore, the crossbeams are made of metal and there is no integration of the concrete flooring with the reinforcing beams. This construction suggests a modular unit that, while fairly light weight, is not as durable arid strong as is needed.
Another disadvantage of the module shown in the Davis patent is the presence of an exposed steel frame. Typical present day building codes require air gaps or a crawlspace separation between grade and a frame. Furthermore, the frames must be protected from moisture and the negative effects of corrosion that are imminent. Steel frames must also include an integral vapor barrier to impede moisture intrusion. The Davis patent shows an exposed steel floor frame, which restricts the ability to set module directly on grade.
U.S. Pat. No. 3,918,222 to Bahramin illustrates another approach to a prefabricated modular flooring and roofing system. This patent describes a floor structure having a plurality of concrete beams or slabs that are pre-stressed longitudinally and arranged in parallel to form a waffle-type structure. Each of the beams or slabs is prefabricated with concrete and includes a pair of longitudinally extending sidewalls. The slabs are reinforced by steel,l reinforcing structures that include a series of longitudinally extending reinforcing rods that are passed through holes formed in the concrete.
Although the Bahramin structure enjoys the strength of concrete beams for lateral and longitudinal support, the structure appears to require large amounts of concrete, which is heavy, and requires substantial manual assembly of the external steel reinforcing structure. Furthermore, this structure, although normally precast off-site, requires a great deal of site work specifically associated with the vertical precast members and the foundations required to support them. As such, this system is not relocatable or modular in nature as an entire system or building assembly.
Another approach is shown in U.S. Pat. No. 4,545,169 to Rizk. This patent describes a monolithic reinforced concrete floor with a frame that includes two longitudinal spaced apart open web trusses and a plurality of quadrilateral tubular beams secured between the trusses in spaced apart relation. A reinforcing mesh material is disposed over the trusses and the tubular beams, and a monolithic concrete slab is formed in situ over the frame encapsulating the reinforcing elements and mesh material.
One significant problem with the Rizk system is the number of different components and elements required to assemble a building. The floor unit, while transportable, does not include integral beams and purlins and relies upon a complex external metal supporting structure that must be preassembled in order to construct a building. Furthermore, this system is also restricted in final location because of the use of an exposed steel frame.
Yet another approach to a prefabricated structure for modular building is found in U.S. Pat. No. 3,811,722 to Jones. This patent describes a bow-shaped foundation structure for mobile homes comprising a pre-stressed lightweight concrete base having longitudinally and transversely extending flanges on the underside which give it structural stiffness and rigidity. Pre-stressed steel reinforcing rods or cables are embedded in the flanges and are bent to obtain maximum vertical force vectors at the ends of the concrete structure.
Although the foundation structure shown in the Jones patent provides structural stiffness for portability, the bow-shaped structure appears to utilize a large quantity of concrete which makes it heavy, and the odd bow shape makes it difficult to adapt for use as a base structure or a roof. Furthermore, the structure requires multiple heights of foundation piers to support the structure in its static final location. This complicated foundation design could be very restrictive for high water table sites and large multi-floor complexes requiring many shared footings. In addition, this foundation may tend to direct surface water under the structure and accumulate water at the footings under the center of the building edges.
Still another approach is shown in U.S. Pat. No. 3,944,242 to Eubank. This patent describes a supporting structure for a mobile home comprising pre-stressed concrete. The supporting structure comprises a rectangular floor supported by beams on the lower surface of the floor. A pair of large longitudinal beams are formed on the lower surface of the floor adjacent to longitudinal edge of the floor. A plurality of smaller transverse beams are formed on the lower surface of the floor connecting the longitudinal beams. Both the longitudinal and transverse beams are pre-stressed. The structure is cast in a single piece in a bed provided with channels for the longitudinal and transverse beams. The beams are pre-stressed in a conventional manner either by pre-tensioning or post-tensioning tendons.
While the supporting structure shown in the Eubank patent provides the advantage of being, portable, the lack of vertical reinforcement in the beams and purlins is a potential problem. Furthermore, this system includes corner supported steel leveling legs and end mounted steel beams for transportation, lifting, and setting of the floor system. Any use of these elements in the final static position of the modular unit, such as leveling throughout the life of the structure, would create a code and corrosion problem. There is no mention of vapor barriers or vapor proofing the assembly, which also limits its building code acceptance.
These and other approaches to modular construction components highlight the persisting need for components that are strong and durable, meet current building codes, yet still be easy to transport. Furthermore, there is a clear need for modular building components that, while still modular and readily transportable to a building site, are more attractive and durable than metal/wood trailer-type modular buildings and therefore facilitate construction of structures that are more likely to remain for long term or permanent use.
Accordingly, there is still a need for a prefabricated concrete-based reinforced floor and roof structure for modular buildings. Especially, there is a need for an improved reinforced structure that provides superior compressive strength in the longitudinal and transverse reinforcing elements, provides space for thermal insulation, does not possess an exposed steel frame, is readily transportable, and provides sufficient strength and light weight for transportability and use both as a floor or as a roof structure.
There is also a need for a prefabricated concrete-based reinforced insulated structure for modular buildings that can be used for walls.